Question: Ishaan did 1 fewer sit-up than Christopher in the evening. Christopher did 21 sit-ups. How many sit-ups did Ishaan do?
Answer: Christopher did 21 sit-ups, and Ishaan did 1 fewer than that, so find the difference. The difference is $21 - 1$ sit-up. He did $21 - 1 = 20$ sit-ups.